In another bowl, whisk sugar, vinegar, vegetable oil, salt, and pepper to combine.
Pour the dressing over the cucumbers and onions.
Refrigerate a couple of hours before serving.
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Recipe Tips
If you are using garden cucumbers that are very moist, you can draw out excess moisture before dressing. Slice the cucumbers, toss them lightly with salt, and let them sit in a colander for 20 to 30 minutes. Rinse, pat dry, then proceed with the recipe. This step is optional but helps the dressing cling to the cucumbers instead of becoming diluted.
Add sugar gradually rather than all at once and taste as you go. The right balance depends on the acidity of your particular vinegar and personal preference.
The salad will release some liquid as it sits in the refrigerator. This is normal and is part of the pickling process. Drain the excess before serving if you prefer a drier texture, or leave it if you want the cucumbers well-saturated with dressing.